METHOD FOR DETERMINING BOREHOLE DIRECTION

Abstract: A method for determining the direction of a borehole during drilling comprises determination of inclination angle theta and highside angle phi from gravity acceleration (g &cir& NOt ) measurements and determination of azimuth angle psi from magnetic field (B &cir& NOt ) measurements, the determinations being carried out in conventional XYZ-and-NEV coordinate systems coupled by Euler-angle coordinate transformations. In particular g &cir& NOt and B &cir& NOt are measured at least at two borehole depths such that phi i NOTEQUAL phi i+1, psi i and psi i+1 being calculated from B &cir& NOt i = [ phi i]<T> [ theta i]<T>{[ psi i]<T>B &cir& NOt e} + B &cir& NOt p and sin<2> psi i + cos<2> psi i = sin<2> psi i+1 + cos<2> psi i+1, with i as number of measurement, B &cir& NOt e as local earth magnetic field, and B &cir& NOt p as perturbating magnetic field. As a result perturbating magnetic fields, for example caused by hot spots or nearby magnetic steel components in the drilling or logging string nearby the B-measuring device, are determined accurately.

Description: METHOD FOR DETERMINING BOREHOLE DIRECTION The present invention relates to a method for determining the direction of a borehole during drilling said borehole. In particular the present invention relates to a method for determining the direction of a borehole during drilling said borehole by using a triaxial accelerometer/magnetometer-package arranged in the drill string employed, said method comprising the steps of: measuring gravity acceleration components g , g , g of the x y z known local gravity acceleration vector g for determining inclination angle θ and highside angle Ψ , and measuring magnetic field components B , B , B of the total magnetic field B for determining azimuth angle -φ , x, y and z indicating vector components in a Cartesian XYZ-coordinate system fixed to said package during said drilling, and , θ, and Φ indicating angles defining rotations between said XYZ-system and a Cartesian NEV-coordinate system, with N the magnetic north direction, V the vertical g-direction, and E the east direction. Such a method is known from US patent 4,163,324. Therein it is demonstrated to use a drill string comprising a drilling bit which is coupled at the one side by a non-magnetic drill collar and at the other side by a set of drill collars made of magnetic material. In turn said set is coupled to a drill pipe. The non-magnetic collar contains a survey instrument, for example a triaxial accelerometer/magnetometer package. When measuring the total magnetic field B, additional to the earth's magnetic field B a perturbating magnetic field B , for example from the above said bit and/or set of drill collars is included. In said patent it is assumed that for the effect of the magnetic drill string the approximation of only a B -vector along the borehole axis Z, being B , is sufficient. Said assumption enables to calculate in a P.z first step an uncorrected azimuth angle, and in a next step to apply an iteration procedure to determine at least a first order correction. In many conditions, however, the assumption of only a B and the approximation of B are far from realistic. P.z P.z For example it is well known that during drilling a non-magnetic collar may become magnetised resulting in so-called hot spots encompassing perturbating magnetic field vectors having unpredictable directions. In US patent 4,682,421 a method for determining a correct azimuth angle by calculating the perturbating erroneous magnetic field M at the location of the instrument is presented. In particular a two-step approach of the above problem is disclosed. After determining the gravity acceleration vector g and measuring the total magnetic field B , which is equal to (B + M) , m e in a first step the cross-axial component M of M is determined. For said first step at least three x-y-measurements are necessary since M is derived graphically from a circle made up of said measurements. Consequently said measurements are carried out by rotating th_ drill string at one location along the borehole axis, being the Z-axis in the measurement coordinate system. It may be clear to those skilled in the art said rotation of the drill string at said location will delay the borehole drilling operation. For the second step in this patent a geometrical determination of M is shown. However, since the application of the cosine-rule (as shown in figure 3 of said patent) for obtaining a minimum error value has to be restricted mathematically to a plane comprising all the relevant parameters including θ and θ n , the determination as presented can only be considered an approximation. Consequently possible errors in M and ψ are dependent on errors in parameters already used in said cosine-rule. Thus, it is an object of the present invention to overcome the problem of rotating the drill string each time the direction of the borehole has to be determined. It is a further object of the present invention to present a method enabling determination of azimuth angles which result from straight forward calculation. It is another object of the present invention to arrive at a method resulting in parameter values which are calculated independently thereby avoiding propagating error calculus . Therefore the method as shown above is improved in accordance with the present invention in that g and B are measured at least at two borehole depths 1. , and 1. . , such that Ψ . ≠ . . , in that φ . y ι' l+l l l+l' *ι and φ . - are calculated in accordance with f its equivalents, with i ■= 1, 2, B being the local earth magnetic field, B T ^ being the magnetic field perturbating B , and [ ] indicating so-called "Transpose" matrices for coordinate transformations from the NEV-system to the XYZ-system under Euler-angles Ψ , θ and φ . In a further embodiment of the present invention g and B are measured at least at three borehole lengths 1., 1. - , and 1. _, 6 l l+l ι+2 In a preferred embodiment of the invention as shown above, a step for checking the outcome of azimuth angles obtained is 2 2 provided in that the (sin " + cos V") ■= 1-equation is verified and compared for every φ . Thus, the invention as disclosed above has the advantage that during drilling the borehole measurement values are obtained in a substantially continuous way, both as to the determination of the borehole direction and to checking the measurement values itself. Consequently irregularities in the measuring process , for example due to unexpected formation conditions or apparatus deficiencies, are traced quickly and reliably. In another embodiment of the present invention the perturbating field B is determined. Advantageously, B obtained results from straight forward calculations thus avoiding approximation procedures, such as there are in iterative processes and graphical determination. The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein: Figure 1 shows the conventional arrangement of an accelero- meter/magnetometer-package within a borehole for measuring g and B with respect to the same Cartesian coordinate frame; Figures 2A and 2B representing the earth reference frame NEV and the tool fixed and package coupled XYZ coordinate frame; Figure 3 shows the generally known principles of the borehole direction and coordinate frame orientations coupled by Euler angle coordinate transformations; and Figure 4 shows schematically the method of measuring during drilling in accordance with the present invention. Referring to figure 1 schematically a surveying instrument to be arranged within a borehole is shown. Said instrument comprises a well-known accelerometer/magnetometer-package for measuring gravity vector components g , g , g and magnetic field vector components B , B , B . The instrument is arranged in such a way that the Z-axis of the instrument is aligned with the borehole Z-axis. Accordingly X- and Y-axes of accelerometer and magnetometer instrument parts are mutually aligned as shown in this figure. In figures 2A and 2B schematically coordinate-frames as used are shown. In figure 2A the earth reference frame NEV is shown, N giving respectively the local magnetic north direction, V the vertical direction, more in particular being the direction of the local gravity vector, and E the east direction, perpendicular to the plane made up by N and V. In figure 2B a Cartesian XYZ-axis is shown, the Z-axis being aligned with the borehole axis. In figure 3 (which can be found e.g. in US 4,163,324) both NEV and XYZ frames are shown with respect to a borehole 1 schematically presented and with respect to each other. As shown in the figure a sequence of three rotations, i.e.: NEV φ > N,E,V θ > N 2 E 1 Z Φ > XYZ, couples vectors in each of the frames, i.e. an azimuth angle φ, an inclination angle θ and a high-side angle Ψ, so-called Euler-angles, which are well-known to those skilled in the art. Said rotations are conventional coordinate transformations represented by matrices , giving for a vector P v and a XYZ NEV formula P NEV = w [θ] [φ] p χγz' or equivalentl y P XYZ " ^ ^ M P NEV With (1) I cos θ 0 sin θ I [θ] = I 0 1 0 (2), and I |_-sin θ 0 cos θ_ I cos Φ -sin Φ 0 I [φ] •= I sin Φ cos Φ 0 (3) , whereas I |_ 0 0 1_ w [θl and [Φ] are the corresponding so-called "Transpose" matrices. As stated above for any P -P -vector couple, the same can be applied on the gravity vector g, being (0,0,g), and B, being (B 0,B ), both in the NEV-frame. 0 T T T Thus, - M [θ] [Φ] 0 (4), and g T T T - [ θ ] 1 [ Φ ] ( 5 ) . V For the specific example of the gravity vector it is noted that the inclination angle θ and the high-side angle Φ can be determined easily for every measurement location as can be read for example in the above-mentioned US 4,163,324. Figure 4 shows schematically the method for determining the direction of a borehole during drilling said borehole. From a rig R at the earth's surface S a borehole b is drilled. For reason of clarity a parallel curve 1 is drawn (as dashed line) for indicating borehole depths (or borehole lengths, or borehole locations) 1 1. , , which are measured along the borehole, with l n at S, at which locations g- and B-measurements are carried out. Schematically, x., y., z., are shown, demonstrating the variable positioning of the survey instrument in the borehole. Furthermore, the perturbating magnetic field B is shown. This B is considered P P dependent on drill string features as explained before, resulting in turn in a rotation and translation of said vector according to the rotation and translation of the XYZ-frame with the survey instrument in the drill string. From the above it may be clear that at every borehole depth or location 1. the total magnetic field B. can be written as 1 ° 1 B. = B + B . However, to calculate this vector sum, a common base l e p or common coordinate frame has to be chosen. As explained above conventionally the XYZ-frame and NEV frame are employed. In order to arrive at the direction of the borehole, besides θ., and Φ . angles, azimuth angles φ . have to be determined. Thereto the above-mentioned vector sum can be expressed as px - «P [ * ] . (6) y py V pz for any borehole depth 1. or measurement number i. From this equation it can be seen easily, that B , B and B are known x y z because they are measured, that the Φ- and θ-matrices are known since Φ and θ are determined in the above-mentioned way, that B and B are known from geomagnetic data bases and that consequently azimuth angle φ and magnetic field perturbation vector components B , B , B have yet to be obtained, p py pz In accordance with the invention for at least two borehole depths 1. and 1. , , which can be written as 1. and 1. , the v l l+l 1 2' components of g and B are measured. Then, for two measurements the following equations are obtained by rewriting the above equation (6): (7) , and 2 B N cos * 2 px I B. [ Φ ] T [θ ] T <-B sin φ r + I B. (8). y2 py z2 pz By well known straight forward calculation of the above equations (7) and (8) it can be seen that the resulting 6 scalar equations for each of the vector components x, y and z, can be considered to comprise 7 unknown parameters, i.e. cos φ , sin φ , cos ώ., sin ψ„, B , B and B ψ 2 ' Y 2 px py pz In order to arrive uniquely at V--* -a d φ„ , as seventh scalar 2 2 2 2 equation sin V>- + cos φ = sin φ„ + cos φ„ is taken. It may be clear to those skilled in the art that also the equivalent 2 2 2 2 equations sin φ 1 + cos V-* ■= 1 , or sin φ„ + cos φ = 1, can be used. It is mathematically self-evident that Φ ≠ , and thus the drill string should have been rotated. Substantially always this criterion is satisfied because the drill string is always rotated between survey location during drilling the borehole. Thus, advantageously the rotations of the drill string usually occurring during the drilling operation, are used, rather than stopping the drilling operation and subsequently rotating as referred to above . After having calculated the values for said 7 parameters φ . -values are obtained in accordance with Based on the same idea, for three measurements at correspondingly three measurement locations, for example 1 1 and 1,, the following equations are obtained two of which being identical to the above (7) and (8) : From the 9 scalar equations which are found by reformulating the above equations (7), (8) and (10), it can be to seen in the same way as shown above that for the 9 unknown parameters the system of equations is complete and no further equations are necessary for solving them uniquely. For the present system of equations cos φ 1, sin V1. > cos V I. sin Ό Z.> cos Ψ i n * sin V- 5- . , Bpx, Bpy and B again can be considered as independent variables. Again φ . -values are obtained in accordance with the above equation (9). Analogously to the case of only two measurements it is noted that Φ ≠ Φ ≠ Φ and no further specific rotation actions are necessary. In a further embodiment of the present invention a check- procedure is comprised. In case of having carried out measurements at two locations 1 and being sin oyed for check purposes. If significant deviations from 1 appear, at a next borehole depth a new set of B and g measurements is taken and the check-procedure can be repeated. Advantageously, also for such a check no additional rotations are required. Again only different highside angles have to be measured. As to the case having carried out measurements at at least three locations and consequently using 9 equations for determining 2 2 azimuth angles φ , φ^ and φ , now sin φ . + cos φ . = 1-equalities , or one of its equivalents being sin φ . + cos φ . = sin φ . ., + cos φ . 1 for respective i-value, are applied for the first time. The same observations are made as to the use and application of said check-procedure. In a next step B can be determined accurately and reliably. In most cases B is coupled to drill string characteristics. Besides such B - determinations sudden changes in B can be traced, P P for example caused by tool failure, magnetic storms, extraneous magnetic fields, etc. As explained above, for the one or the other determination procedure, only two or three measurement sets repectively are required. It may be clear that normal operation conditions cover several thousands of feet or several kilometers borehole depths and a plurality of measurement sets are obtained. Consequently borehole directions can be determined and followed quickly and reliably without special operational effort. Various modifications of the present inventions will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims: C L A I M S 1. A method for determining the direction of a borehole during drilling said borehole by using a triaxial accelerometer/magneto- meter-package arranged in the drill string employed, said method comprising the steps of, - measuring gravity acceleration components g , g , g of the known local gravity acceleration vector g for determining inclination angle θ and highside angle Φ and measuring magnetic field components B , B , B of the total magnetic field B for determining azimuth angle φ ; x, y and z indicating vector components in a Cartesian XYZ- coordinate system fixed to said package during said drilling, and φ , θ and Φ indicating angles defining rotations between said XYZ-system and a Cartesian NEV-coordinate system, with N the magnetic north direction, V the vertical g-direction, and E the east direction wherein g and B are measured at least at two borehole depths 1. and 1. ., , such that Φ . ≠ Φ . - , in that φ . and r l l+l l l+l l φ and , or one of its equivalents, with i = 1, 2, ..., B being the local earth magnetic field, B being the magnetic field perturbating B , and τ P e [ ] indicating "Transpose" matrices for coordinate transformations from the NEV-system to the XYZ-system under Euler-angles Φ , θ, and Φ - 2. The method as claimed in claim 1, further comprising the steps of: 2 2 checking if said equivalent (sin φ . + cos φ . ) is equal to 1, measuring g and B at least at one further borehole depth 1. 2 2 if (sin φ . + cos φ . ) ≠ 1, with Φ . ≠ Φ . , ≠ Φ . -, v *ι l l l+l ι+2 calculating V'- . . an< ^ carrying out a next checking step. 3. A method for determining the direction of a borehole during drilling said borehole by using a triaxial accelerometer/magneto- meter-package arranged in the drill string employed, said method comprising the steps of: - measuring gravity acceleration components g , g , g of the known local gravity acceleration vector g for determining inclination angle θ and highside angle Φ ; and measuring magnetic field components B , B , B of the total magnetic field B for determining azimuth angle φ , x, y and z indicating vector components in a Cartesian XYZ- coordinate system fixed to said package during said drilling, and φ , θ and Φ indicating angles defining rotations between said XYZ-system and a Cartesian NEV-coordinate system, with N the magnetic north- direction, V the vertical g-direction and E the east direction, wherein g and B are measured at least at three borehole depths 1., 1. n and 1. _, such that Φ . ≠ Φ . . ≠ Φ . . , in that φ . , r l l+l ι+2 l l+l ι+2 *ι ' φ . , and φ . „ are calculated in accordance with coordinate transformations from the NEV-system to the XYZ-system under Euler-angles Φ , θ and φ . 4. The method as claimed in claim 3, further comprising the steps of: 2 2 checking if sin φ . + cos φ.= 1 for at least one i or one of its equivalents ; measuring g and B at least at one further borehole depth 1. if sin 2 φ . + cos2 φ . ≠ 1, with Φ . ≠- Φ . - ≠ Φ . _ ≠- Φ . _ ; l l l l+l 1+2 ι+3 calculating φ . _, and - carrying out a next checking step. 5. The method as claimed in any one of the claims 1 to 4, wherein the perturbating magnetic field B is determined.

Inventors: NICHOLSON JAMES WILLIAM, NICHOLSON, JAMES, WILLIAM
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1994
Main classification: E21B47
IPC codes: E21B47